Heat exchanging apparatus

ABSTRACT

A heat exchanging or circulation apparatus comprising a system of conduits connected to an inlet and an outlet for circulating water or other practically incompressible liquid through the system, heat being transferred through the conduit walls, circulation through the apparatus being periodically shut-off, whereupon continued heat transfer through the conduit walls causes freezing of the liquid to ice in the conduits. Two first portions of the system are relatively heat insulated or shielded from flowing cold air to obtain delayed freezing of the water in these portions in relation to the freezing of the liquid to ice in uninsulated second portions of the system located between the first two portions so that ice growing in the second portions towards the ends thereof will be in communication with the two first portions will result in an increased pressure on the unfrozen liquid in the insulated portions of the system, the increased water pressure being relieved by the two first portions each connected through insulated branch conduits with a closed insulated pressure relief or absorbing means so as to avoid rupture of the conduits in any portion of the system.

The present invention relates to a heat exchanging apparatus forcirculating or conducting heated water through conduits swept by air tobe heated. The invention relates particularly to improvements of a heatbattery in the form of pipes in a duct leading air from outside toinside of a building, and to improvements of radiators.

Installations of water-air heat exchangers of different kinds, heatbatteries in air conditioning equipment, ordinary water radiators etcoften have problems with pipe ruptures due to freezing in low airtemperatures. Attempts to achieve reliable protection against piperupture due to freezing in such installations have not been successfulso far. The heavily dimensioned pipes have not been able to withstandthe heavy compression forces occurring when ice forms in the pipingsystem. Pipe rupture in heat batteries of the type illustrated in FIG. 1usually occur at the pipe bends, and for preventing freezing of theseportions, they have been further insulated against the cold air flowingthrough the battery. These measures have been unsuccessful, however, fora reason which will be clearly apparent below.

Attempts have also been made to sense the temperature at the placeswhere pipe rupture usually occurs. When the temperature approaches 0° C.at the sensors, the flow rate is automatically increased by a regulatingunit. These attempts have also been unsuccessful for the same reasonwhich will be explained below.

Within the industry, these problems with rupture due to freezing havebeen regarded for some time as more or less insoluble.

The present invention thus has the object of achieving a heat exchangerof the types mentioned above, that is heat batteries and radiators,which is protected against pipe rupture, should ice formation occur inthe piping. The heat exchanger should be reliable, maintenance-free andfunction without electronic or other sensors. This is achieved by a heatexchanger of the type described in the opening paragraph of claim 1 andhaving the features set forth in the characterizing clause thereof.

The solution which the present invention signifies is partly based on adiscovery completely incompatible with the generally acceptedunderstanding as to how pipe rupture during freezing occurs, and onwhich all the previous attempts to provide a satisfactory solution havebeen based. Tests carried out by me under controlled conditions in aresearch laboratory have namely shown that pipe rupture during freezingdoes not occur at the ice plug formd, but at a part of the pipe wherethe water is not yet frozen. The pipe rupture customarily occurs due tothe increasing pressure in the still unfrozen water due to a growing iceplug somewhere else in the pipe. This explains whytemperature-controlled frost-protection means have not been able tosolve the problem. It is not possible to measure the temperatureeverywhere in the circulation system. The pipe rupture occurs where thewater is warmest, and it is here that temperature sensors have beenplaced. Reliable temperature sensing in the unprotected heat-exchangingparts of the pipes is not possible due to the widely varyingtemperatures between the pipe fin surfaces, which are subjected toflowing cold air and the interior of the pipe. Furthermore, the sensorshave a reaction time which is too long in the rapid freezing process.

This situation, that pipe rupture takes place at a part of the pipewhere the water has not yet frozen, has avoided discovery due to anotherscarcely noted property of water, namely that its freezing point fallswith increasing pressure. Growing ice plugs increase the pressure in theas yet unfrozen part, simultaneously as the temperature can fall below0° C. in the still unfrozen water. When the pipe subsequently bursts,the pressure falls suddenly and the freezing point is instantaneouslyraised to 0° C. again, the water immediately freezing to ice. In mostcases the repairman is confronted with a protruding ice plug at theplace of rupture, and draws the conclusion that the pipe was poorlyinsulated at this particularly place since a bursting ice plug hasobviously been formed there. This generally accepted "knowledge" as tohow pipe rupture occurs has merely led one skilled in the art tosolutions, e.g. extra insulation, which have made the problem worserather than solved it.

Due to this discovery I have been able to attack the problem with acompletely different understanding and have achieved a solution which issimple, reliable, completely maintenance-free and easy to apply inexisting structures. It has also made it possible to use thinner copperpipes and thereby increase the heat conductive (cold take-up) ability ofthe uninsulated pipe parts.

The present invention will now be described in greater detail withreference to some examples, illustrated in the accompanying drawings.

FIG. 1 schematically illustrates in cross-section a conventional heatbattery provided in a duct for leading cold air from outside to inside abuilding;

FIG. 2 schematically illustrates the above known heat battery improvedin accordance with the present invention;

FIG. 3 is an enlarged sectional view of the upper left corner of FIG. 2;

FIG. 4 is a front view of a radiator forming a heat exchanger accordingto the invention;

FIG. 5 is an end view of the radiator in FIG. 4;

FIG. 6 is an enlarged sectional view according to line 6--6 in FIG. 4and FIG. 7;

FIG. 7 shows an alternative embodiment of the radiator in FIG. 4;

FIG. 8 is an end view of the radiator in FIG. 7;

FIG. 9 is an enlarged sectional view according to line 9--9 in FIG. 7;and

FIG. 10 shows a further alternative embodiment of a radiator as a heatexchanger according to the invention.

The conventional heat battery 10 illustrated in FIG. 1 is located in aspace 10A in a building and is used in an air conditioning installationfor heating fresh outdoor air which is blown by a fan through a duct 11and past the uninsulated parts 12 of the pipe system, which leads thehot water from a district heating network, heating unit or the like, thehot water entering an inlet 13 and leaving through an outlet 14. Thepipe bends 15 are usually not subjected to the cold air and are thusrelatively insulated. Should water circulation take place too slowly orcompletely cease for some reason, ice plugs can be formed in theuninsulated, unprotected pipe parts 12 and rapidly increase the pressurein the insulated pipe bends 15, leading to pipe rupture there. Piperupture in the bends can, for example, occur after some few minutes invery cold weather if the circulation pump were to stop and the fan tocontinue blowing cold air through the installation. Even if the fan isautomatically shut down when circulation is poor, the air flows continuedue to so-called "downdraft".

FIG. 2 illustrates a heat battery 10A in accordance with my invention,where each pipe bend 15 is in communication with a collecting chamber 16and a pressure chamber 16A. The collecting chamber 16 and the branchconduits 17 between this chamber and the pipe bends 15 areheat-insulated. The branch conduits or pipes 17 are restricted to adiameter of only 2-3 mm, in order not to disturb the water circulationin normal operation. The water in the piping system is normally under apressure of 200 kPa and the air in the pressure chamber 16A is thereforeunder the same pressure of 200 kPa. If ice plugs are formed in theuninsulated pipe portions 12, the pressure in the pipe bends 15increases when the ice plugs grow. This pressure is taken up by thecompressible air in the pressure chamber 16A and thus prevents the piperupture which otherwise would occur. Even if all the water in the heatbattery were to freeze to ice, the pressure never goes above 600 kPa,which is far below the rated pressure for ordinary copper pipes. In thisconnection it is important that the pipe bends 15, the restricted branchconduits 17, the tube-like collecting chamber 16 and the pressurechamber 16A are relatively insulated, to be quite sure that the waterthere freezes last. The principle of the invention can also be appliedto other types of heat exchangers, such as radiators, where thecirculation is kept going, although ice plugs have been formed in someof the pipe coils.

It is of course possible within the scope of the invention to use otherpressure-relieving means than a pressure chamber with an enclosed gascushion, e.g. different kinds of safety valves, and to utilize theinvention in completely different connections, where pipe rupture due tofreezing occurs, e.g. in buried water pipes or pipes in buildings, wherethe pipes transfer heat to the surrounding soil or air. In such anapplication of the invention, when the buried pipe is frozen, the iceplug grows in both directions and reaches an area where the water pushedby the ice plug enters a collecting duct connected to a pressure chamberwith means permitting the pressure to rise to a predetermined value butwell below that value which would result in pipe rupture.

In order to facilitate the understanding of the invention, reference ismade to FIG. 3.

The pipes or conduits 12 are provided with flanges 12A.

In both pipes an ice plug 18 is growing towards the pipe bend 15. Inknown pipe systems this would rapidly increase the pressure of the waterin th pipe bend to a value which would result in rupture.

According to the invention, the pipe bends 15, branch pipes 17, thecollecting chamber 16 and the pressure chamber 16A are allheat-insulated by means of heat insulating material indicated byreference numeral 19, which will prevent water in these members tofreeze. Relative insulation of these elements can also be achieved bysimply shielding them from the cold air to which the other pipe surfacesare exposed. Accordingly, the water is allowed to flow slowly under thepushing action from the growing ice plugs 18.

In the pressure chamber the water level rises from the normal level 20to level 21, which results in a compression of the air in the space 22.

The pressure chamber 16A may be preloaded with a gas under relativelyhigh pressure supplied through a valve 23.

There may also be provided a safety valve 24 which opens at apredetermined pressure.

Alternatively, the pressure chamber 16A may be filled with water, and inthis case the safety valve 24 admits water to be discharged at apredetermined pressure.

In FIG. 4 is shown a conventional radiator 25 with vertical waterchannels 26 connecting a lower collecting chamber 27 with an upprcollecting chamber 28.

An upper pressure chamber 29 and a lower pressure chamber 30 is dividedinto two compartments by a separating wall 31.

Each of the compartments is connected to the adjacent pressure chamber29 and 30, respectively, through an insulated branch pipe 32, into whichice plugs 18 may grow and press the water into the chamber 29, therebypreventing rupture of the conduits of the system.

FIG. 7 shows a modified radiator 25A relative to the radiator in FIG. 6.The lower pressure chamber 30 is omitted, and instead the outermostvertical water channels 33,34 have been heat-insulated by means of heatinsulating material 19 as shown in FIG. 9.

FIG. 10 shows another conventional radiator 35 having parallel pipes 36,insulated pipe bends 37, insulated branch pipes 38, insulated collectingchambers 39,40 and insulated pressure chambers 41,42 substantiallyarranged as in the embodiment shown in FIG. 2.

The heat battery 10A in FIG. 2 and the radiator 25 in FIG. 4 have beentested down to -20° during long and repeated test periods without anyrupture in the pipe system. The invention has therefore proved to bevery useful and efficient in practice.

I claim:
 1. Heat exchanging or circulation apparatus comprising a systemof conduits connected to an inlet and an outlet for circulating water orother practically incompressible liquid through the system, heat beingtransferred through the conduit walls, circulation in said apparatusbeing periodically shut off, whereupon continued heat transfer throughthe conduit walls causes freezing of the liquid to ice in the conduits,characterized in that two first portions of said system are relativelyheat-insulated or shielded from flowing cold air to obtain delayedfreezing of water in said portions in relation to the freezing of theliquid to ice in uninsulated second portions of said system locatedbetween said first two portions so that ice growing in said secondportions towards the ends thereof being in communication with said twofirst portions will result in an increased pressure on the unfrozenliquid in said insulated portions of the system, said increased waterpressure being relieved by having said two first portions each connectedthrough insulated branch conduits with a closed insulated pressurerelief or absorbing means so as to avoid rupture of the conduits in anyportion of the system.
 2. An apparatus according to claim 1,characterized in that the respective pressure chamber at one end formsan expansion chamber in which a gas volume forms a compressible gascushion to permit water pushed by the ice in said second portion toenter the expansion chamber under increased counter-pressure from thegas cushion.
 3. An apparatus according to claim 2, characterized in thatthe expansion chamber is preloaded with a gas at a predetermined highpressure above the normal water pressure in the system when it is shutoff and before freezing.
 4. An apparatus according to claim 1,characterized in that the pressure chamber is provided with a safetyvalve which opens at a predetermined water pressure to prevent ruptureof the conduits.
 5. An apparatus according to claim 1, characterized inthat the conduit system is provided in a duct for conveying air fromoutside to inside a building, the conduits being pipes provided withheat exchanging flanges and extending across the duct within the sameand being connected with each other through relatively insulated orshielded connecting pipe portions without flanges and lying outside theduct at opposite sides thereof in the air space in the building, eachconnecting portion at the respective side of the duct being connected tosaid insulated pressure chamber through said insulated branch conduits.6. An apparatus according to claim 1, characterized in that saidconduits are incorporated in a radiator.
 7. An apparatus according toclaim 6, characterized in that the radiator has vertical water channelsbetween a lower collecting duct and an upper collecting duct, the upperduct being connected though insulated branch conduits with a pressurechamber.
 8. An apparatus according to claim 7, characterized in that thelower collecting duct is connected through insulated branch conduits toa lower pressure chamber.
 9. An apparatus according to claim 7,characterized in that the outermost channels connecting the twocollecting ducts are heat-insulated to delay freezing to ice therein.10. An apparatus according to claim 1, characterized in that theconduits are pipes running back and forth in parallel-spacedrelationship to form heat exchanging pipes in a radiator, the parallelpipes being pairwise interconnected by insulated connecting portions,which through insulated branch pipes are connected with an insulatedpressure chamber.